1. Field of the Invention
The present invention relates to a display device using an organic light-emitting element and a manufacturing method thereof. More specifically, the present invention relates to a sealing structure for protecting an organic light emitting element deposited onto a substrate, and to a display device, in which an element substrate (a substrate having at least an element such as a TFT) and a sealing substrate are provided in the proximity, and a manufacturing method thereof.
Throughout the specification, the term “organic light-emitting element” indicates an element including an organic compound film sandwiched between two electrodes to achieve light emission. One of the representative organic light-emitting elements is a light-emitting element using an organic light-emitting diode (OLED). The OLED includes an organic compound film sandwiched between two electrodes. Holes are injected to one of the electrodes while electrons are injected to the other electrode. The holes and the electrons are therefore coupled with each other to cause light emission.
2. Description of the Related Art
In recent years, display devices using the organic light-emitting element are actively studied. The display devices using the organic light-emitting element can be reduced in weight as well as in thickness as compared with a conventional CRT, and thus their application to various uses is attempted. Portable telephones and personal digital assistants (PDAs) can be now connected to the Internet, resulting in a remarkable increase in the amount of information to be displayed as images. Accordingly, there arises an increasing demand for display devices to perform color display and to enhance their definition.
On the other hand, light weight is an important factor for the display device included in such a portable digital assistant. For example, a portable phone having less than 70 g in weight is now on the market. In order to reduce the weight of the portable digital assistant itself, a throughout review as to the weight of almost all the used components such as individual electric components, a body and a battery is performed. In order to realize a further reduction in weight, however, it is necessary to lighten the display device itself.
Since a display device including a pixel portion formed with an organic light-emitting element is a self-light emitting type display device, a light source such as a backlight is not required as is needed in liquid crystal display devices. Therefore, such a display device using the organic light-emitting element is promising means of realizing light weight and a thin body.
The organic light-emitting element is capable of emitting blue light and thus allows the realization of a full-color display self-light emitting type display device. However, various deterioration phenomena are observed in the organic light-emitting element. It is urgently required to eliminate such phenomena as obstacles to the practical use of the organic light-emitting element.
For example, the dark spot, which is a non-light emitting point defect appearing in a pixel portion, is regarded as a problem that remarkably reduces the display quality. It is said that the dark spot is a progressive defect and the mere presence of water increases the number of dark spots even if the organic light-emitting element is not operated. It is believed that the cause of the dark spot is a high reactivity of a cathode containing an alkali metal or an alkali earth metal with water or oxygen.
Therefore, in the display device using the organic light-emitting element, a desiccant is placed in a sealed region surrounded by an element substrate, a sealing substrate and a sealing agent so that water and oxygen do not enter the organic light-emitting element. FIG. 15 is a cross-sectional view showing a display device using a conventional organic light-emitting element. An element substrate 301 made of glass, on which an organic light-emitting element 307 is provided, and a sealing substrate 303 provided so as to be opposed to the element substrate 301, are bonded to each other through a sealing agent 302 so that the organic light-emitting element is prevented from being exposed to the outside air. A fine protective film 308 having a thickness of 100 nm to 500 nm may be further provided on the organic light-emitting element 307 so as to prevent water from entering the organic light-emitting element 307. A sealed region is filled with dry air. A distance between the element substrate 301 and the sealing substrate 303 is adjusted by mixing a filler, a spacer or the like into the sealing agent 302.
The sealing substrate 303 has a hollow region made of a metal such as stainless steel or aluminum, which is processed into a dish-like shape. A desiccant 304 and a film sheet 305 are provided in the hollow region. The desiccant 304 has moisture-absorption characteristics so as to absorb water penetrating into the sealed region to prevent the degradation of the organic light-emitting element. If the desiccant 304 gets into the display area where the organic light-emitting element 307 is provided, display performance of the device is deteriorated. Thus, the film sheet 305 having a permeability to gas and vapor is bonded to the sealing substrate 303, thereby confining the desiccant 304 within a concave portion of the sealing substrate 303. The film sheet 305 has a thickness of 100 to 300 μm. In consideration of flexion of the film sheet 305 generated by the weight of the desiccant 304, a gap between the film sheet 305 and the organic light-emitting element 307 where they are bonded to each other so as not to be in contact is required to be 50 to 200 μm. Accordingly, with the film sheet 305 provided in the hollow region of the sealing substrate 303, the hollow region is required to have a depth of at least 150 to 500 μm. The placement of the film sheet 305 or the like increases the distance between the element substrate and the sealing substrate, making it difficult to reduce the thickness of the display device. In view of this problem, the present invention has been made and a first object of the present invention is to provide a display device, in which the element substrate 301 and the sealing substrate 303 can be provided in close proximity to each other so as to enable the reduction in thickness of the device, and a manufacturing method thereof.
Conventionally, in order to provide the hollow region in the sealing substrate where the desiccant is placed, it is required to use a sealing substrate made of a metal material that is easy to process. In the display device including the sealing substrate made of metal, however, the element substrate 301 made of glass can be uniquely used as a substrate through which light emitted from the organic light-emitting element is output. Thus, if thin film transistor (TFT) elements are provided on the element substrate, light emitted from the organic light-emitting element is obtained from the side of the element substrate through the TFT elements, resulting in lowered brightness of the emitted light. Moreover, the thinner the element substrate becomes, the lower the shock resistance becomes. As a result, the element substrate is likely to be fragile. In particular, when the sealing substrate made of metal and the element substrate made of glass are bonded to each other, the distortion is generated by a sudden change in temperature due to the difference in thermal expansion coefficient, causing the cracking in the element substrate.
Accordingly, a second object of the present invention is to provide a display device achieving a bright display with good visibility by increasing the brightness of light emitted from the organic light-emitting element and a manufacturing method thereof. In addition, a third object of the present invention is to provide a display device, in which breakage due to a sudden change in temperature is restrained and a manufacturing method thereof.
Furthermore, the sealing agent placed on the side faces of the display device is made of an organic resin material, which has a high moisture permeability as compared with inorganic type glass materials and metal materials. For example, the organic resin material has a permeability of 15 g/m2 for 24 hr. to 30 g/m2 for 24 hr. at 60° C. and a humidity of 90%. Although the amount of water penetrating into the sealed region through the element substrate made of glass from the front side of the display device and the amount of water penetrating into the sealed region through the sealing substrate made of a metal material from the rear side of the display device are negligibly small, water which enters the sealed region through the sealing agent having a high moisture permeability from the side faces of the display device causes the deterioration of the organic light-emitting element. Thus, it is necessary to take measures to cope with this problem.
Since the amount of water passing through the sealing agent is determined by the product of the area of the sealing agent exposed to the outside air multiplied by the moisture permeability of the sealing agent, it is preferred that the area of the sealing agent exposed to the outside air is as small as possible, that is, the sealing agent is as thin as possible. However, the sealing agent serves not only to bond the sealing substrate and the element substrate to each other, but also to control the distance between the sealing substrate and the element substrate. Thus, it is necessary to determine the thickness of the sealing agent in consideration of the distance between the element substrate and the sealing substrate so that the element substrate and the sealing substrate do not come in contact with each other to break the organic light-emitting element provided on the element substrate and the transistors that make a current flow through the organic light-emitting element.
In view of the above problem, a fourth object of the present invention is to provide a display device with enhanced reliability, in which the element substrate and the sealing substrate can be provided close to each other and the amount of water vapor passing from the side faces of the element device through an organic resin material such as a sealing agent to enter the sealed region is reduced so as to prolong the lifetime of the organic light-emitting element; and a manufacturing method thereof.
Alternatively, in order to reduce the amount of water entering from the side faces of the display device, a protective film having a low permeability to a fine gas and water vapor may be considered to be provided between the sealing agent and the outside air so as to contact the sealing agent. However, the formation of the protective film on the side faces of the sealing agent by using a vacuum apparatus after bonding the element substrate and the sealing substrate to each other through the sealing agent leads to an increase in manufacturing cost, thereby decreasing the advantages of the organic light-emitting element which is easily manufactured at a low cost. In addition, since a stress is applied onto a material that is applied onto the substrate by a dispenser system so as to cure the sealing agent, the sealing agent after curing has a gently undulating shape in the thickness direction between the substrates on the side faces of the sealing agent as well as in the width direction perpendicular to the thickness direction of the side faces of the sealing agent. It is difficult to form the protective film on such side faces of the sealing agent, whereby providing a part where the protective film is not formed.
As described above, it is difficult to reduce the amount of water passing through the sealing agent provided on the side faces of the display device. Accordingly, in the display device using the organic light-emitting element, there is a great need to reduce water passing through the side faces of the display device. Consequently, the fourth object of the present invention is important.
In addition to the appropriate realization of the first to fourth objects, the present invention enables the reduction in the number of paths through which water passes from the outside air and the control for making the distance between the element substrate (first substrate) and the sealing substrate (second substrate) uniform.